1. Field of the Invention
The present invention relates to an improvement of a bearing and a screw compressor and more particularly to a bearing capable of detecting a temperature and a screw compressor having the bearing. Especially, the present invention is concerned with an oil cooled screw compressor of the type in which oil after separation and recovery from discharge gas is supplied as lubricating oil.
2. Description of the Related Art
A screw compressor has a casing for housing therein a pair of female and male screw rotors meshed with each other. End portions of rotor shafts of each of the pair of screw rotors are supported by radial bearings. On the other hand, in each of the pair of female and male screw rotors, a pair of tilting pad thrust bearings for bearing a thrust force developed on the screw rotor are mounted at the end portion of one rotor shaft. The tilting pad thrust bearings are disposed on both sides of a disc-like thrust member fitted on the end portion of one rotor shaft in each of the pair of screw rotors and are provided with plural pads (hereinafter referred to as “thrust bearings”) which, in a sliding contact with a slide surface of the thrust member, undergo the thrust force transmitted from the screw rotor to the thrust member. In the screw compressor of the above construction, since the screw rotors are rotated at high speed, plain bearings are used as the radial bearings and the thrust bearings. Since the slide portions of the bearings are required to be low in friction, a low friction material is used particularly as the material of slide portions of the thrust bearings which receive a thrust force from the thrust members of a high peripheral velocity.
As a thrust bearing using a low friction material as the material of its slide portion, one constituted for example by a plural-layer slide member to be described is known. In this plural-layer slide member, a porous intermediate layer is formed between a metallic backing strip difficult to bond and a resin layer as a slide member to let an anchoring effect be exhibited, thereby making the metal layer and the resin layer difficult to peel from each other and improving the abrasion resistance and sliding characteristic of the resin layer. More particularly, as a slide layer there is used a layer formed by adding 10 to 95 wt % of a molten fluorine resin of tetrafluoroethylene-perfluoroalkylvinyl ether polymer (PFE) or polytetrafluoroethylene-hexafluoropropylene polymer (FAP) to polytetrafluoroethylene (PTFE). To this slide layer are added a material for improving the abrasion resistance and a material for improving the sliding characteristic (see, for example, Japanese Patent Laid Open No. 2002-194380).
The temperature of a bearing is based on heat produced in the thrust member or a shaft and the sliding portion of the bearing. Usually, an oil film is present between the thrust member or the shaft and the slide portion of the bearing and functions to remove heat, so that the temperature of the bearing does not rise beyond a certain level. However, if the oil film breaks or the thrust member or the shaft and the bearing come into direct contact with each other, the temperature of the bearing itself rises rapidly into an abnormally high temperature, resulting in damage of machine having such bearings, thus leading to a serious accident such as a stop of operation. To avoid this inconvenience, there is known a conventional example in which a temperature sensor is embedded in a bearing itself and, when the temperature detected by the temperature sensor exceeds a certain level, it is judged that the bearing is in an abnormal condition. “Thrust bearing trouble detecting device and method” related to this second conventional example will be described below with reference to FIG. 12 which is a construction diagram of the thrust bearing trouble detecting device.
A rotary disc 52 is disposed on a lower surface of a thrust collar of a rotary shaft 51, and plural fan-shaped bearing segments 54 disposed radially around the rotary shaft 51 are supported by a segment support base 55. The rotary disc 52 and the bearing segments 54 are accommodated within an oil reservoir 61 filled with lubricating oil. An oil film 53 is formed between the rotary disc 52 and the bearing segments 54, and the rotary disc 52 is supported sidably by the bearing segments 54 through the oil film 53. The surfaces of the plural bearing segments 54 which are in sliding contact with the rotary disc 52 are formed using a lining material of a synthetic resin. Temperature sensors 62 for detecting the temperatures of the bearing segments 54 are attached to all of the plural bearing segments 54. Temperatures Ta to Tz of the bearing segments 54 measured by the temperature sensors 62 are inputted to a bearing temperature monitor 64 through a temperature transducer 63, in which when any of the temperatures Ta to Tz exceeds a predetermined value, it is judged that the bearing is in an abnormal condition (see, for example, Japanese Patent Laid Open No. 2002-168242).
The “thrust bearing trouble detecting device and method” relating to the second conventional example described in Japanese Patent Laid Open No. 2002-168242 is considered to be extremely useful because a trouble of the thrust bearing can be detected by temperature measurement. However, when such a construction is applied to the bearing related to the first conventional example wherein the slide portion is formed by a resin layer, even if there occurs a rise in temperature due to breaking of the oil film or due to direct contact of the thrust member or shaft with the bearing. However, there sometimes is a case where the temperature is not transmitted to the temperature sensor and a trouble of the bearing cannot be detected, because the resin is much lower in thermal conductivity than the metal.
There also is known an oil cooled screw compressor having a pair of intermeshing female and male screw rotors housed within a casing and supported by radial bearings and thrust bearings to which oil after separation and recovery from discharge gas is supplied as lubricating oil. This oil cooled screw compressor (a conventional example) will be outlined below with reference to FIG. 17 which is a schematic explanatory diagram showing the whole of the compressor.
The reference mark C1 in FIG. 17 denotes an oil cooled screw compressor according to the conventional example. In the oil cooled screw compressor C1, an oil separating/recovering unit 83 is disposed in a discharge flow path 82 formed in a screw compressor body 81. A lower portion of the oil separating/recovering unit 83 is formed as an oil sump 84. From the bottom of the oil sump 84 there extends an oil supply flow path 86 which passes through an oil cooler 85 and reaches a rotor chamber and bearings/shafts sealing portions (neither shown). On the other hand, the portion of the discharge flow path 82 located on a secondary side of the oil separating/recovering unit 83 extends from an upper portion of the oil separating/recovering unit 83.
In the oil cooled screw compressor C1, compressed gas is supplied to the rotor chamber and the bearings/shafts sealing portions and, together with lubricating oil which has been conducted to a discharge port, is discharged toward the oil separating/recovering unit 83, in which gas/liquid separation is performed. The thus-separated lubricating oil is once stored in the oil sump 84, while the compressed gas which has become clean after the separation of the lubricating oil is discharged to the portion of the discharge flow path 82 which extends from the upper portion of the oil separating/recovering unit 83.
A layer L of lubricating oil separated and recovered from the compressed gas is formed at all times in a lower portion within the oil sump 84. A lubricating oil viscosity affecting component which forms a layer U on an upper surface of the lubricating oil layer L without being dissolved in the lubricating oil is discharged from the oil separating/recovering unit 83 under a discharge pressure to a drainage flow path 88 by opening an on-off valve 87 at appropriate time intervals. The lubricating oil which forms the lower layer L is supplied to the rotor chamber and the bearings/shafts sealing portions through the oil supply flow path 86 and is thereafter conducted again to the oil separating/recovering unit 83 through the foregoing discharge port (see, for example, Japanese Patent Laid Open No. 2000-186688).
In conventional oil cooled screw compressors, white metals or aluminum alloys are widely used as the materials of bearings which support screw rotors (see, for example, R&D KOBE STEEL TECHNICAL REPORT, Vol.49, No.1, APRIL 1999 page 33).
Although a detailed description is not found in the above laid open patent 2000-186688 and R&D KOBE STEEL TECHNICAL REPORT, for example as shown in FIG. 18 which is an explanatory diagram showing in what state a rotor shaft is fitted in a radial bearing, an oil groove 72b is formed in a surface of a radial bearing 72 opposed to a rotor shaft 71, i.e., in an inner periphery surface of the radial bearing 72. Further, an oil supply hole 72a is formed in the radial bearing 72, the oil supply hole 72a communicating with the oil groove 72b from an outer periphery of the radial bearing 72 to supply oil (lubricating oil) to between the inner periphery surface of the radial bearing 72 and the outer periphery surface of the rotor shaft 71. In the case of an oil cooled screw compressor, the pressure of oil supplied to the radial bearing 72 is almost equal to the discharge pressure. In many cases, gas is dissolved in the oil. The pressure of oil discharged from the radial bearing 72 is almost equal to the suction pressure in the oil cooled screw compressor. However, the pressure of oil present in the oil groove 72b of the radial bearing 72 is an intermediate pressure between the discharge pressure and the suction pressure in the oil cooled screw compressor.
That is, in the oil groove 72, the pressure of oil is reduced from the pressure equal to the discharge pressure in the oil cooled screw compressor to the intermediate pressure between the discharge pressure and the suction pressure, so that the gas dissolved in the oil separates at the time of the pressure reduction and there occurs what is called foaming. The oil which is in such a foaming state is rolled up onto the rotor shaft 71 and is supplied in a large quantity to a pressure receiving surface 72c on which a load W is imposed, resulting in that an oil film present on the pressure receiving surface 72c of the radial bearing 72 breaks. Consequently, the rotor shaft 71 and the radial bearing 72 come into direct contact with each other and there arises a fear of occurrence of a mechanical trouble such as galling or scratching of the shaft and the bearing. Although reference has been made above to the radial bearing as an example, the above description is also applicable to a thrust member and a thrust bearing which bear a thrust force of each rotor shaft in an oil cooled screw compressor.
The foregoing Japanese Patent Laid Open No. 2000-186688 discloses, for the protection of bearings, etc. in an oil cooled screw compressor, an effective means which uses a simple construction for separating a lubricating oil viscosity affecting component in an oil separating/recovering unit and which thereby conducts only lubricating oil having a viscosity of an allowable value or higher to the bearings, etc. However, it does not disclose any means for eliminating the trouble caused by the foregoing foaming of oil.